But despite the “great advances” on the possible motives, police said they still have no idea who the killer might be, except that he was an "experienced gunman".

The brother, a “nominated suspect”

On September 5, 2012, three members of the British-Iraqi al-Hilli family were shot dead in their BMW station wagon in a woodland car park close to the village of Chevaline in the hills above Lake Annecy.

A French cyclist Sylvain Mollier was also found shot dead at the scene.

The two al-Hilli daughters, then aged four and seven, survived the attack in which more than 20 bullets were fired. The oldest daughter, Zainab, 7, was shot in the shoulder and beaten around the head, leaving her with a fractured skull while her 4-year-old sister, Zeena, hid for eight hours under the skirt of her dead mother in the back of the family's car and survived without any injuries.

Zaid al-Hilli, Saad al-Hilli’s older brother, remains the main suspect. British detective superintendent Nick May said: "Zaid al-Hilli is a nominated suspect. He has been arrested and we continue to make enquiries.”

Family feud over a multi-million-dollar inheritance

Both French and British investigators said that recorded phone conversations have lead police to believe a family feud could be behind the tragedy.

Maillaud told reporters that the two brothers were locked in a bitter dispute over properties in Iraq, London and elsewhere, as part of a multi-million-dollar inheritance left by their father after he died in 2011.

"This is being followed up actively," Maillaud said, adding that the two brothers "initially worked together to regain their father's property" in Iraq but soon fell out.

"The question arises if those in Iraq who are in control of the father's property do not have an interest in making the two brothers disappear because they want to get it back," he said.

"There is a tremendous amount of work that needs to be done...several audios to be digested," May added. "There are other lines of enquiry."

Foreign states and industrial espionage with secret service involvement

Maillaud evoked the possibility that another explanation for the murder could lie in Saad al-Hilli's career as "a brilliant engineer" working with a British firm specialising in satellites which worked with numerous foreign countries.

"If you are talking about foreign states and industrial espionage, you could also be talking about secret service involvement," Maillaud said.

"It is an extremely complex aspect of the investigation, which will take a lot of time and perhaps will not lead anywhere. But this avenue of investigation is far from being concluded."

Maillaud reiterated that the investigators did not think French cyclist Mollier was in any way involved, repeating his belief that he had simply been "in the wrong place at the wrong time."

Most mysterious case in 20 years

Speaking on Wednesday, Kevin Hurley, the police and crime commissioner in Surrey, England -- where Saad al-Hilli lived with his family -- said that the case was the “most mysterious he had come across in 20 years,” British newspaper the Independent reported.

Overall, more than 100 police officers in France and Britain have been involved in the case, and 800 witnesses have been interviewed.